Recently, in regard to prevention of the atmospheric pollution, tightening of the automotive exhaust gas regulation has been discussed.
At present, a catalytic converter having a ceramic honeycomb structure as a carrier, which is monolithically formed by extrusion, and has a number of passages (cells) each surrounded by partition walls, is used for purification of automotive exhaust gas. In order to improve purification efficiency of the honeycomb structure, a study is in progress on improvement of so called warm up characteristic, that is improvement of catalytic activity at an initial driving stage by reducing a heat capacity of the honeycomb structure, consequently by making warm up faster.
It is required to reduce weight without changing geometrical surface area of a honeycomb structure (that is to reduce bulk density of a honeycomb structure) for reducing heat capacity of a honeycomb structure. Thus, actions such as reducing a thickness of partition walls, increasing porosity, or the like are taken to meet the requirement. For example, reduction of the honeycomb partition walls thickness is extremely difficult in the production technology, because in general it accompanies a decline of the mechanical strength of the honeycomb structure. Since the amount of the kneaded clay in the outer wall part of the honeycomb structure (honeycomb formed body) to be obtained at the time of extrusion forming differs from that of the partition walls, the outer wall and the partition walls are inevitably unbalanced. If the outer wall is made thicker, the cell deformation defect occurs, and if it is made thinner, fissure (crack) of the outer wall occurs. Therefore forming of the outer wall part of the ceramic honeycomb structure is the key point of the production technology.
Therefore, in the case of such a honeycomb structure, the open defects including a fissure and a crack extending through the outer wall, for example, various defects extending through the outer wall as shown in FIG. 1 (open tears 1, lateral fissure 2, vertical fissure 3) occur inevitably in a certain ratio in drying or firing steps.
Such an open defect including a fissure and a crack extending through an outer wall occurred in a production process may cause a problem in a catalyst coating process. In the catalyst coating process, an aqueous slurry including a mixture of γ-alumina for increasing the specific surface area, ceria as an oxygen absorbing material, zirconia for improving the heat resistance, or the like, is supplied to the honeycomb passage with a pressure applied by sealing the outer circumferential part of the ceramic honeycomb structure. At the time, if there is a hole extending through the outer wall, when a positive pressure is applied to the aqueous solution, a part of the aqueous slurry leaks to the honeycomb outer circumferential part, thereby the leaked portion of the slurry is wasted and a sealing trouble would occur at the time of the next process. On the other hand, in the case the honeycomb structure end face is kept at a negative pressure, such a trouble that the slurry is not coated at least partially on the surfaces of the honeycomb passages would occur.
Moreover, the honeycomb catalyst obtained in the catalyst coating process is used in an airtight metal container. Since the thermal expansion of the metal container is larger than the thermal expansion of the honeycomb catalyst, the honeycomb catalyst is housed in the metal container via a ceramic mat having the cushion property with preloaded mechanical pressure thereon so as to avoid loosening even the metal container heated in use expands more than the honeycomb catalyst. Here, if the catalyst material locally adheres to the outer wall of the ceramic honeycomb catalyst, the mechanical pressure at that portion becomes high, and could destroy the honeycomb catalyst in some cases.
To solve the above-mentioned problems, various countermeasures are proposed, such as, a honeycomb structure covered with a thick reinforcing material at outer circumferential part (see the patent literature 1:JP-B-51-44713), a honeycomb structure with an outer wall provided on the structure without an outer wall, cut out from a formed and fired honeycomb structure (see the patent literature 2:JP-Y-53-34373), a honeycomb structure with the outer circumferential wall with a glaze applied thereto (see the patent literature 3:JP-U-53-133860), one a honeycomb structure having the passage of the outer circumferential part filled with a ceramic material (see the patent literature 4:JP-A-56-129042), a honeycomb structure with reduced porosity at the outer circumferential part including the outer (see the patent literature 5:JP-A-56-129043), a technique for obtaining a honeycomb structure having a good dimensional accuracy by adding a ceramic material to the outer circumference of the structure having a poor accuracy (see the patent literature 6:JP-U-7-183), and a honeycomb structure having incomplete cells in the outermost circumference closed selectively with a ceramic material (see the patent literature 7: JP-A-7-246341).
However, the prior art disclosed in patent literatures 1 to 7 has the problems mentioned below. According to the patent literature 1, although the defect of the outer wall can be repaired, the thermal shock resistance is lowered because the coefficient of thermal expansion of the reinforcing material is higher than that of the cordierite honeycomb structure. Moreover, in the case the outer wall is covered with the reinforcing material, the increased heat capacity retards the temperature rise of the catalyst, and consequently the purifying performance at the time of cold start deteriorates.
The patent literature 2 is directed to the technique for applying an outer wall to a honeycomb structure without an outer wall, cut out from a formed and fired honeycomb structure, and thus it is not a technique for closing a large through hole (through hole having for example 0.1 mm or more diameter) extending through the outer wall.
According to the patent literature 3, which is directed to the technique for applying a glaze to the outer circumferential wall, a defect such as a large hole having a 0.1 mm or more diameter and a relatively deep split, or the like cannot be filled. Moreover, since the coefficient of thermal expansion of the glaze is higher than the coefficient of thermal expansion of the honeycomb structure, the thermal shock resistance deteriorates.
According to the patent literature 4, although the outer wall defect can be repaired indeed, the part with the cells in the outer circumferential part filled becomes a dead zone with respect to the exhaust gas purification as the purpose of the catalyst, because the filled part does not have the exhaust gas flow. In these days the honeycomb structure is supported only the side surface supported mechanically in consideration of using the entire volume, the part of the filled passage is totally wasted according to this technique. For example, in the case of a Φ100 mm catalyst, if a 3 mm width dead zone is provided, about 10% or more volume is wasted.
According to the patent literature 5, the pores of a honeycomb structure are filled. The pore of an ordinary ceramic honeycomb structure is mainly 50 μm or less. Therefore, this technique does not fill a defect such as a large hole having a 0.1 mm or more diameter and a relatively deep split.
The patent literature 6 is directed to the technique for obtaining a honeycomb structure having a good dimensional accuracy by covered the external circumference of the structure having a poor accuracy with a ceramic material. In some cases, there would be a part not covered with the ceramic material. Moreover, as in the case of the patent literature 1, since a 0.5 mm or more covering layer is partially present, deterioration in purification property at the cold start cannot be avoided.
The patent literature 7 is directed to the technique for selectively closing the incomplete cells in the outermost circumference with the ceramic material. Since the open defect including a fissure and a crack extending through an outer wall (see FIG. 1) is not taken into consideration at all, although the defect can be closed if the defect is at an incomplete cell part by chance, a defect at a complete cell part cannot be closed.